1. Field of the Invention
The present invention relates to a semiconductor laser used in the fields of optical communications, optical recording, optical information processing, and the like, and a method of making the same.
2. Related Background Art
In the field of long-distance optical communications, there has been a demand for semiconductor lasers which can continuously emit light for a long time with a high output. When a semiconductor laser having such a characteristic is available, the number of optical repeaters each containing the semiconductor laser therein can be reduced, and the semiconductor lasers may be replaced less frequently. However, it is difficult for the conventional semiconductor lasers to continuously emit light for a long time with a high output. One of the reasons thereof is the melting breakdown of the light-emitting end face upon a high output. This failure suddenly occurs during a high-output continuous light-emitting operation of the semiconductor laser, thereby causing a crucial damage to the semiconductor laser. Accordingly, it has been known as Catastrophic Optical Damage (COD). In particular, the COD frequently occurs in semiconductor lasers whose oscillation wavelength is 1.1 .mu.m or shorter. While semiconductor lasers with an oscillation wavelength of about 0.98 .mu.m has been expected to be utilized as a light source for optical fiber amplifiers such as EDFA (erbium-doped fiber amplifier), it is difficult for currently available semiconductor lasers in this oscillation wavelength region to continuously emit light for a long time with a high output, thus keeping long-distance optical communications by means of such optical fiber amplifiers from becoming practicable.
In order to suppress the COD, the density of nonradiative recombination centers between the end face of the semiconductor laser active layer and the reflecting layer may be lowered or restrained from increasing. In a so-called window structure, a semiconductor layer having a wide energy band gap, which is transparent to light of the oscillation wavelength, is formed on the above-mentioned end face and functions as a nonabsorptive region in the vicinity of the end face, thereby restraining the nonradiative recombination centers from increasing and the end face from deteriorating. Also, a current non-injectable region may be formed in the vicinity of the end face of the active layer so as to reduce the current leaking to the end face, thereby preventing the nonradiative centers from increasing at the end face. Further, as disclosed in the U.S. Pat. No. 5,144,634, there has been proposed a semiconductor laser whose end face is coated with a ultrathin film having a thickness of several nm.
Also, Japanese Patent Application Laid-Open No. 65986 discloses a method in which an element identical to that contained in the active layer is introduced through the end face thereof so as to render a higher resistivity to the end face.
In the semiconductor lasers with any of the configurations mentioned above, it is necessary to effect cleavage in a vacuum or provide the end face with an additional semiconductor layer, thus complicating the apparatus for making the same. Also, the semiconductor laser made by means of such a method does not seem to have a high reproducibility. Further, when an element identical to that contained in the active layer is introduced through the end face thereof, although the vicinity of the end face attains a higher resistivity, thus manufactured semiconductor laser may not sufficiently emit light for a long time with a high output in a continuous manner.
In order to overcome such problems, it is an object of the present invention to provide a semiconductor laser which can continuously emit light for a long time with a high output, and a method of making the same.